1. Field of the Invention
The present invention relates to a method and apparatus for measuring the optical quality of a reflective surface, for example, measuring the flatness of a sheet of glass that could contain a defective region where the surface is slightly curved.
2. Background and Technical Field of the Invention
Production of tempered glass consists of heating the material to its softening temperature, then fast chilling, to introduce compressive surface stresses and increase its strength. In this process, the hot material is supported and moved in and out of the heating chamber by a set of rolls. As result of the combined actions of sag between the rolls and roller eccentricity, the glass sheet deforms slightly, acquiring a surface waviness, also called Roller Wave, as shown in FIG. 1. When installed in a building, glass exhibiting this waviness will generate distortion of reflected images and be considered defective.
Several tools and methods are presently used to inspect tempered glass. With reference to FIG. 1 showing a sheet of glass 10 having a waviness, the simplest measuring tools include a depth gage revealing the depth w of the wave as a difference between peak 12 and valley 13 heights of the glass 10. The depth w of the wave, however, does not fully describe the optical distortion.
Other methods use optical means to quantify the optical distortion. With reference to FIG. 1A, devices such as that in U.S. Pat. No. 3,857,637 to Obenreder measures an angle B of a reflected beam of light 16 off of the surface 18 of the glass 10 using a beam-position sensing device 20. The reflected beam comes from a light source LS providing a beam of light 14 directed at the surface 18. This approach requires measuring the angle B of reflection at two or more points 22a, 22b, and recording the variation of the reflected angle B and the distance d between the measured points 22a, 22b, to permit the calculation of the optical distortion.
U.S. Pat. No. 5,251,010 to Maltby discloses a method that eliminates the need of measuring the distances d such as that illustrated in FIG. 1B. Maltby discloses methods whereby two parallel beams of light, which can be split from a single light source LS with partial mirrors 23 as shown, separated by a known distance d, are reflected off the inspected surface 18 and sensed by two position-sensing devices 20a,20b. As result of the curvature due to the roller wave, these two beams 16a,16b diverge or converge, and the change in the angle B between these beams provides the measure of the distortion.
A nearly identical device is described in U.S. Pat. No. 5,122,672 to Mansour and U.S. Pat. No. 5,210,592 to Betschneider. The two-beam approach requires very accurate beam-position detectors and does not account for the difficulties in measuring the beam position when the beam shape becomes irregular as result of the surface curvature. Another approach, described in Redner & Bhat, “New Distortion Measuring Method Using Digital Analysis of Projection-Moire Patterns” SAE Transactions, 106 (6) 1997, uses the image of a Moiréscreen projected on a master, forming Moire fringes that reveal changes in magnification due to local curvature of the inspected item. The application of this method is also described in U.S. Pat. No. 5,128,550 to Erbeck. Another method based on measurements of the dimensional size of the reflected beam is described in U.S. Pat. No. 4,585,343 to Schave. In this method, edges of the reflected beam are located using an array of detectors. This method is essentially equivalent in performance to the two-beam method in U.S. Pat. No. 5,251,010 discussed above since the distance between the two edges of the reflected beam is used to measure angular changes. More recently, an approach proposed in U.S. Pat. No. 6,100,990 to Ladewski uses reflected images of gray-scale patterns. Assuming that the roller wave is periodic in nature, the distribution of light intensity analysis permits calculation of the optical power of the inspected surface.
All of the above methods have serious limitations, conceptual or practical in nature. Common difficulties include the following:
a) The measured angular deviation B is very small, and the detection of small changes in the reflected beam position cannot be accomplished accurately considering that the glass sheet vibrates as it emerges from the tempering furnace, and
b) The surface curvature deforms the beam of light, making it difficult to locate its center using a position-sensing device.
For at least these reasons, a new method and apparatus that overcomes the limitations of the prior methods and apparatuses is desirable.
One objective of the invention is to provide for measuring optical distortion more accurately, eliminating the reflected beams position-sensing detectors used in the above described devices. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.